I won't buy Nikon out of the US due to the warranty issue, but I have successfully purchased an excellent Sigma lens from B&H. It arrived to me on Friday after I sent the first copy back due to a defect. The return process was painless, though I did have to pay duty again. It xomes with a one year international warranty - pro grade glass, total of about $600 CAD (incl tax, duties, shipping) vs. about $800 (w/tax). I am unsure about Canon's policies as I have not had occasion to look in to it, though I suppose that it would be simlar to Nikon's if the pricing strategy is the same. Nikon Canada will not even touch US or grey market lenses for repair from what I have heard, even if you are willing to pay fo it.
I picked up the Sigma 18-50 2.8 DC EX HSM Macro and it is fantastic. The closest equivalent Nikon is about $1400 CAD.